[Q] How to modify mic pre-amp to higher gain?

Discussion in 'The Projects Forum' started by zrbarnes, May 1, 2011.

  1. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    I'm currently trying to modify the input of a Velleman MK140 audio kit to accept an electret condenser style microphone (generic computer mic). The kit comes ready to accept a dynamic mic. I could build a simple condenser preamp and feed it into the input, but I thought it might be better if I just modded the current input circuit.

    The audio quality does not have to be great, so the simplest solution is probably the best.

    Does anyone have any pointers?

    I've attached the schematic that I need to modify below.
     
  2. beenthere

    Retired Moderator

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  3. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    As of this point, I don't have the original 1/4" plug attached, I have a mono 1/8" plug that I used for verifying that I need more gain (can't really hear anything). Positive side of the mic is connected to the negative side of C3 (from the schematic above), and the negative side is connected to ground.

    I have 12v, 5v, and 3.3v buses available, if needed.

    The link you posted seems like it packs all the info I need, but deciphering the easiest way to adapt it into my circuit is currently going over my head. (Fighting a head cold...)
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    You can still use your mono plug. Just connect a bias resistor from V+ (on your preamp) to the input side (-) of C3.
     
  5. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    Oh yeah, I have stereo jack to use I guess too, if that's necessary.
     
  6. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    Ok, I think I finally get it. Apply the voltage to the positive side of the mic, and I may not need to actually change the gain of the preamp circuit. Makes more sense now, and I will test it out tomorrow.

    I noticed tonight, when I hooked an electret microphone capsule thing directly into the input of the circuit that when I first turned it on, for 3-4 seconds I could hear myself as it faded back to silence. Then, it would happen for about a 1/4 second when I turned the power off (when capacitors are charging/discharging, correct?). I don't see anything physically wrong; checked the polarity of all the caps and all that, and tried to double check everything.

    Any ideas where to look? I'd rather not damage hearing/equipment every time I turn the power on/off.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    With your present circuit, charging up the input coupling capacitor at switch-on may supply a momentary bias to the electret microphone. With a proper resistor connection, permanent bias can be supplied.

    To avoid an excessive "pop" at power up/down, you might want to put a second resistor in series with the positive feed to the bias resistance = perhaps a quarter of the value. Return the junction of the two resistors to common via a capacitor of 100μF or so, and the bias voltage will appear less suddenly.
     
    Last edited: May 2, 2011
  8. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    Alright, did some tests today and it seems like I'm close to the end.

    A couple more questions though:

    I have switched to a stereo jack, as shown in the schematic... good or bad idea? Should I short pin 2 and 3 (right now, pin 2 is not connected)?

    The internally mounted pot I changed because I didn't like the 22k one I had.

    If anyone could take a look at the schematic and tell me how bad I messed it up :D, that would be great!

    Also, this kit was designed for a 9v battery. I have 12v, 5v, and 3.3v buses available. What would be the recommended way to get the appropriate voltage? (Or can it run off of 12v w/o significant risk?)
     
    Last edited: May 2, 2011
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    No, replace the wire from Vcc to your input with a 10K resistor and see how it works.
     
  10. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    A potentially dumb question here: would it be better (or even just about the same) to swap out R15 (560 ohm) with a 10k resistor?
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Like i said this morning on the other website, the circuit uses an inverting opamp with a low input impedance for a dynamic mic (coil and magnet) which is too low for an electret mic but you did not talk about an electret mic. Increasing R15 to 10k will be good for an electret mic but because the opamp is inverting then the gain of the first opamp drops from a max of 84 to only 4.7.
    Your red wire shorts the output of an electret mic.

    Since the circuit is designed for a 150 ohm to 300 ohm dynamic mic then use a dynamic mic with it.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    If you want to use your electret then I would recommend replacing R15 with a 10K and testing with the electret mic. This will produce a gain of only 2. If you need more gain then replace RV1 with a 200K. This will give IC1B a max gain of 20.

    Gain = RV1/R1
    If RV1=10K & R1=10K Then Gain = 1
    If RV1=100K & R1=10K Then Gain = 10
    If RV1=500K & R1=10K Then Gain = 50
    If RV1=1M & R1=10K Then Gain = 100

    Edit: "Gain" refers to the gain of IC1B only.
     
    Last edited: May 3, 2011
  13. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    Unfortunately, I must use an electret based mic (generic computer condenser microphone) for my current project, so it is necessary for me to modify the circuit to suit it.

    All labels (R15, RV1, R2, V+, etc) used below are referring to the label used on the schematic posted above:

    I've been busy tonight so I haven't gotten to play around with it, but I was able to hear myself this morning when testing R15 = 1k ohm, RV1 = ~40k ohm, R2 = 4.7k ohm, V+ connected to pin 3, GND connected to pin 1, and pin 2 unconnected. It was faint though, and I'm pretty sure it was cutting off a lot of the treble. By the way, hooking it up this way did get rid of the 3-4 seconds of increased volume after turning on the circuit that I mentioned earlier (the kit still pops when you turn it on, however it's not caused by the mic preamp, but elsewhere).

    I'm going to bump R15 up to 10k ohm to match the high input impedance of the electret mic, and then a larger pot to RV1 and see what it sounds like, then go from there.

    Still don't know if I should short pins 2 and 3 on the input jack, leave pin 2 floating, or if there is some other better solution (on the schematic I posted, the red dashed line just means that I thought that maybe I should connect pins 2 and 3).

    Thanks guys, I know this is probably boring stuff, so I appreciate the help.
     
    Last edited: May 3, 2011
  14. Audioguru

    New Member

    Dec 20, 2007
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    Pin 3 on the mic jack is probably the mic signal. DO NOT short pin 3 to V+ (as shown by your red wire) but instead connect a 10k resistor to pin 3 with the other end of it at V+ to power the electret mic.
    We don't know if pin 2 of the mic jack does anything (maybe for an earphone?) so leave it disconnected.
     
  15. zrbarnes

    Thread Starter New Member

    May 1, 2011
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    Gotcha. I will do exactly that. 10k resistor between V+ and pin 3.

    After doing that, do I remove the R15 resistor, or should I still do as said before and swap it out with the 10k? Does C3 stay where it is?

    I also may see if I can figure out the pin 2 vs. pin 3 thing too. They may actually be shorted inside the mic for all I know, because they both seemed to work as inputs.
     
    Last edited: May 3, 2011
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    I must have had a brain fart when I said 10K. A more realistic value would be 1 to 2K.
    Replace it with a 10K. C3 stays but since your input impedance will now be 10K you can reduce its value to ~1uF if you want to. Probably not worth the effort though.

    Is your electret a 2 or 3 terminal model?

    http://www.epanorama.net/zen_schematics/Prac/ecm.html
     
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